Method and circuit for image-in-image overlay

ABSTRACT

The invention relates to a method and a circuit arrangement for picture-in-picture insertion, in which a sequence of insertion pictures (Kj=K1,K2, . . . ) is read, with vertical decimation (VD≧1), into a memory device (S) and subsequently read out, the sequence of insertion pictures (Kj) read out is inserted into a sequence of main pictures (Hi=H1, H2, . . . ) and the memory device (S) is continuously overwritten by the insertion pictures. 
     In order to prevent the occurrence of a seam during the insertion of the insertion pictures into the main pictures in a cost-effective manner and with a relatively low outlay on apparatus, 
     the memory device (S) is subdivided into memory segments (X,Y,Z) which are continuously cyclically overwritten by the insertion pictures, 
     the memory device (S) has a storage capacity of less than two insertion pictures, and 
     a decision is made as to whether the currently written insertion picture (Kj) or the immediately preceding insertion picture (Kj−1) is read out.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for picture-in-picture insertion andto a circuit arrangement for picture-in-picture insertion.

2. Description of Related Art

In the case of such picture-in-picture (PIP) insertion, a smallerinsertion picture (small picture) is inserted into a larger mainpicture. The insertion picture is decimated in accordance with the sizereduction and is continuously read into a memory device, older storedpictures being overwritten, and then the insertion pictures are read outin a manner employing synchronization with the main pictures.Accordingly, the read-out speed of the insertion pictures is generallyhigher than the writing speed. In this case, the main picture and theinsertion picture may, in a known manner, be fields which are used fordisplaying a monitor picture.

At specific phase angles of the rasters of the main picture and of thesmall picture, the higher read-out speed can lead, inter alia, to theread-out pointer overtaking the write pointer and reading out a previouspicture stored in the memory device, with the result that a seam occursin the middle of a displayed small picture and in part the precedinginsertion picture is read out. If both insertion pictures originate fromdifferent motion phases, a disturbing effect results since movingobjects through which the seam runs are displayed with distortion. Ifthe frequencies of the pictures of the insertion channel and mainchannel correspond only approximately, the result is slow drifting ofthe disturbance location, which is found to be particularly unpleasant.

EP 0 739 130 A2 describes a method for eliminating this seam by storingtwo fields of a small picture, with the result that the field that canbe read is always exactly the one which is currently not being written,and, consequently, the read pointer cannot overtake the write pointer. Afirst and a second memory, which each store a field, are provided forthis purpose. This method has the disadvantage, however, that a storagecapacity of two insertion pictures or fields is necessary, which entailscorresponding costs.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the invention is based on the object of providing a methodand a circuit arrangement for picture-in-picture insertion with whichthe occurrence of a seam in the insertion picture can be prevented in acost-effective manner and with a relatively low outlay on apparatus.

The invention is based on the concept that it is not necessary, inprinciple, to store two whole insertion pictures in order to prevent thewrite pointer from being overtaken by the read pointer. Instead of usinga storage capacity of two insertion pictures, a smaller memory device issubdivided into a suitable number of segments, and suitabledecision-making is effected to stipulate whether the currently writtenor the preceding insertion picture is read out.

Consequently, according to the invention, in contrast to the use of twoseparate memory segments for the currently written and the precedinginsertion picture, if appropriate even the currently written insertionpicture is read out if it is ensured that the read pointer does notovertake the write pointer.

For this purpose, memory segments which, in particular, are the samesize can be cyclically overwritten in a predetermined order since, inparticular, good periodicity of the operation can also be ensured bythis means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below using a number ofembodiments with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of a circuit arrangement according to theinvention;

FIGS. 2 a, 2 b, and 2 c are illustrations of a memory device accordingto a first embodiment of the invention; and

FIG. 3 is an illustration of a memory device according to a secondembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with FIG. 1, a sequence of main pictures Hi=H1, H2, H3, .. . are output from a main picture source 1 via a main picture channel 7to a control device 3. Correspondingly, from an insertion picture source2, a sequence of insertion pictures Kj=K1, K2, K3, . . . decimated by adecimation device 12, i.e. reduced in size relative to the mainpictures, are output to a memory device S and buffer-stored. In thiscase, both the main pictures Hi and the insertion pictures Kj are fieldswhich are combined e.g. in a line-offset manner to form the overallmonitor picture. Afterward, the sequence of small pictures is read outand forwarded to the control device 3 via an insertion picture channel8. If an asynchronous main picture source 1 and insertion picture source2 are used, the read-out operation of the memory device S is effected ina manner exhibiting synchronization with the main pictures Hi. Onaccount of the decimation, in particular the vertical decimation, theread-out of the insertion pictures Kj from the memory device by thecontrol device 3 takes place more rapidly than the operation of writingto the memory device. The control device 3 combines the main pictures Hiand insertion pictures Kj to form an overall picture which is reproducedon a monitor 6.

One-quarter (¼) picture-in-picture insertion is assumed below, where thesmall picture is correspondingly decimated in each case by the factor 2in the horizontal and vertical. According to the invention, it isprovided for this purpose that the memory device has a storage capacityof 1.5 fields (decimated relative to the main pictures) and, inaccordance with FIGS. 2 a, 2 b, and 2 c, is subdivided into three memorysegments X, Y and Z, all three segments being the same size, i.e. eachhaving a storage capacity of 0.5 field (decimated relative to the mainpictures) and being continuously overwritten in this cyclic order.Consequently, a writing start segment I and a second writing segment IIare in each case required for a field.

Accordingly, in a first storage operation in accordance with FIG. 2 a, amemory area formed from the start writing segment X and the secondwriting segment Y is written to the first field K1. The second field K2is correspondingly written to the start writing segment Z and the secondwriting segment X in the subsequent storage operation in accordance withFIG. 2 b, the start segment of the first field K1 already beingoverwritten in the process of writing to the second writing segment X.During the third storage operation, the field K3 is correspondinglywritten to the start writing segment Y and the second writing segment Zin accordance with FIG. 2 c. Consequently, at the instant when, in FIG.2 c, the write pointer is located in the start segment Y, in thelocation designated by SZ1, the second half of the first field K1 isoverwritten, i.e., at this instant, K1 is still partly present, K2 iscompletely present and K3 is currently being written to the segment Y.

For the read-out operation it must be ensured that, on the one hand, awhole field is read out and, on the other hand, the read pointer doesnot overtake the write pointer. For this purpose, a decision must bemade as to whether the currently written field Kj or the immediatelypreceding field Kj−1 is read out. Since the difference in the writingand read-out speed is essentially determined by the vertical decimationVD, where VD is a natural number, the decision as to which field is tobe read out can be made dependent in each case on VD and on the positionof the write pointer in the currently written field.

This position of the write pointer generally depends on the position ofthe small picture in the main picture and hence primarily on the phaseangle of the small picture and main picture, the read pointer generallybeing fixedly coupled to the main picture via the insertion position.

In the case of a vertical decimation of VD=2, the read-out speed isabout twice as high as the writing speed, with the result that the readpointer would overtake the write pointer in the currently written fieldif the write pointer has only written less than half of the field, i.e.is still located in the start segment, as is the case in the positionSZ1 in FIG. 2 c during the writing of the field K3. Consequently, thepreceding field K2 must be read out in this case, i.e. the reading startsegment is the writing start segment I of the previous field, i.e. thesegment Z according to FIG. 2 b. By contrast, at the position SZ2 inFIG. 2 c, at which the write pointer is already located in the secondwriting segment Z, the writing start segment Y can be taken as thereading start segment.

Generally, it can be derived from these considerations that 2*VD−1segments, each having a storage capacity which corresponds to thequotient of the storage capacity required for an insertion picture andVD, are necessary in order to ensure in each case that either thecurrently written or the immediately preceding insertion picture can beread out. The total memory space required is thus (2−1/VD) times thestorage capacity required for an insertion picture. The saving incomparison with the use of two memory areas for a respective insertionpicture thus falls with increasing vertical decimation VD. Since thequotient of reading speed and writing speed can, to a goodapproximation, be applied as VD, the decisive criterion for theselection of the reading start segment is whether the last segmentrequired for writing the current insertion picture is already beingwritten to.

In the case of 1/9 picture-in-picture insertion, VD=3 and, in accordancewith FIG. 3, it is necessary correspondingly to choose 2*VD−1=5 segmentsA, B, C, D and E each having a storage capacity of ⅓ field, with theresult that a total storage capacity of 5/3 fields is required. In thiscase, too, memory segments I, II, III are cyclically overwritten, withthe result that the first field is written to the segments A, B and C,the second field to the segments D, E and A, etc. Since the readingspeed is about three times higher than the writing speed, the decisioncriterion to be applied here is whether more than 1/VD=⅓ of the memoryspace required for a field remains to be written to. Consequently, inthis case, too, the resulting decision criterion is whether the lastsegment, in this case the third segment III, required for the currentfield is already being written to.

In addition to the elimination of the seam, it is furthermore possibleto eliminate disturbances that may arise as a result of different fieldpositions in the insertion channel 8 and main channel 7, e.g., in thecase of a picture composed of line-offset fields, disturbances betweenthe upper field in the main channel 7 and the lower field in theinsertion channel 8. This can be ensured e.g. by storing an additionalline, with the result that the lines of the upper field of the insertionchannel, despite the dependence on the raster position of the field ofthe main channel, are always displayed relatively above the lines of thelower field of the insertion channel.

1. A method for picture-in-picture insertion, wherein a sequence ofinsertion pictures decimated by vertical decimation are read into amemory device and subsequently read out, wherein the insertion picturesread out are inserted into a sequence of main pictures, wherein thememory device has a storage capacity of greater than one insertionpicture but less than two insertion pictures and is subdivided intomemory segments which are continuously overwritten by the insertionpictures, wherein a decision is made as to whether the currently writteninsertion picture or the immediately preceding insertion picture is readout, wherein more than one memory segment of the memory device isrequired for storing an insertion picture, and in that the memorysegments of the memory device are cyclically overwritten by theinsertion pictures in a predetermined order, and wherein in a mannerdependent on the ratio of a reading speed of a read pointer to a writingspeed of a write pointer and a relative position of the write pointer ina writing area holding the currently written insertion picture, adecision is made as to whether the currently written insertion pictureor the immediately preceding insertion picture is read out.
 2. A methodfor picture-in-picture insertion, wherein a sequence of insertionpictures decimated by vertical decimation are read into a memory deviceand subsequently read out, wherein the insertion pictures read out areinserted into a sequence of main pictures, wherein the memory device hasa storage capacity of greater than one insertion picture but less thantwo insertion pictures and is subdivided into memory segments which arecontinuously overwritten by the insertion pictures, wherein a decisionis made to whether the currently written insertion picture or theimmediately preceding insertion picture is read out, wherein more thanone memory segment of the memory device is required for storing aninsertion picture, and in that the memory segments of the memory deviceare cyclically overwritten by the insertion pictures in a predeterminedorder, and wherein the memory device has a storage capacity which is(2−1/VD) times the storage capacity required for an insertion picture,where VD is the vertical decimation of the insertion picture.
 3. Themethod of claim 2 wherein the memory of segments are the same size andthe number of memory segments 2*VD−1, the number of memory segmentsrequired for an insertion picture corresponding to the verticaldecimation (VD).
 4. The method of claim 3 wherein a memory segment has astorage capacity of 1/VD times the storage capacity required for aninsertion picture and the decision criterion that is applied is whetherthe last memory segment required for the currently written insertionpicture is already being written too.
 5. The method of claim 2 whereinthe insertion pictures and main pictures are fields of a monitorpicture.
 6. A method for picture-in-picture insertion, wherein asequence of insertion pictures decimated by vertical decimation are readinto a memory device and subsequently read out, wherein the insertionpictures read out are inserted into a sequence of main pictures, whereinthe memory device has a storage capacity of greater than one insertionpicture but less than two insertion pictures and is subdivided intomemory segments which are continuously overwritten by the insertionpictures, wherein a decision is made as to whether the currently writteninsertion picture or the immediately preceding insertion picture is readout, wherein more than one memory segment of the memory device isrequired for storing an insertion picture, and in that the memorysegments of the memory device are cyclically overwritten by theinsertion pictures in a predetermined order, and wherein a comparison ismade to determine whether a main picture and an insertion picture to beinserted into the latter have identical field position, and, in the caseof a differing field position, an identical field position is achievedby address shifting of the main picture or of the insertion picture. 7.A circuit arrangement for picture-in-picture insertion, comprising: amemory device for storing vertically decimated insertion pictures, thememory device having a storage capacity of greater than one insertionpicture but less than two insertion pictures and being subdivided intomemory segments which can be continuously overwritten by the insertionpictures; a control device for reading out the vertically decimatedinsertion pictures from the memory device and for inserting theinsertion pictures read out into a sequence of main pictures; and adecision device for deciding whether the currently written insertionpicture or the immediately preceding insertion picture is read out,wherein each memory segment has a storage capacity of less than oneinsertion picture, the memory segments of the memory device can becyclically overwritten by the insertion pictures in a predeterminedorder, and wherein the memory device has a storage capacity which is(2−1/VD) times the storage capacity required for an insertion picture,where VD is the vertical decimation of the insertion picture.
 8. Thecircuit arrangement of claim 7 wherein the memory segments are the samesize and the number of memory segments is 2*VD−1, the number of memorysegments required for an insertion picture corresponding to the verticaldecimation (VD).
 9. A circuit arrangement for picture-in-pictureinsertion, comprising: a memory device for storing vertically decimatedinsertion pictures, the memory device having a storage capacity ofgreater than one insertion picture but less than two insertion picturesand being subdivided into memory segments which can be continuouslyoverwritten by the insertion pictures; a control device for reading outthe vertically decimated insertion pictures from the memory device andfor inserting the insertion pictures read out into a sequence of mainpictures; and a decision device for deciding whether the currentlywritten insertion picture or the immediately preceding insertion pictureis read out, wherein each memory segment has a storage capacity of lessthan one insertion picture, the memory segments of the memory device canbe cyclically overwritten by the insertion pictures in a predeterminedorder, and wherein in a manner dependent on the ratio of a reading speedof a read pointer to a writing speed of a write pointer and a relativeposition of the write pointer in a writing area holding the currentlywritten insertion picture, the decision device decides whether thecurrently written insertion picture or the immediately precedinginsertion picture is read out.